1. Field of the Invention
The invention relates to a rotation angle detection device that detects the rotation angle of a rotor.
2. Description of Related Art
A brushless motor used in, for example, an electric power steering system is controlled by applying currents to stator coils according to the rotation angle of a rotor. FIG. 4 shows, by way of example, a conventional rotation angle detection device. This rotation angle detection device includes a rotor 1 that has a magnet with two poles, that is, a north (N) pole and a south (S) pole, and two magnetic sensors 11 and 12 that are arranged such that the angular interval, about the rotation axis of the rotor 1, between the magnetic sensors 11 and 12 is 90 degrees. The magnetic sensors 11 and 12 output respective sinusoidal signals that have a phase difference of 90 degrees. The rotation angle detection device detects the rotation angle of the rotor 1 based on the two sinusoidal signals. With regard to such a rotation angle detection device, refer to, for example, Japanese Patent Application Publications No. 2008-241411, No. 2002-213944, No. 2010-110147, No. 2006-78392, and No. 2010-48760.
The direction indicated by the arrow in FIG. 4 is the normal rotation direction of the rotor 1. The rotation angle of the rotor 1 increases as the rotor 1 rotates in the normal direction, and decreases as the rotor 1 rotates in the reverse direction. As to the rotation angle θ of the rotor 1, the magnetic sensor 11 outputs an output signal V1 expressed as V1=A1·sin θ, and the magnetic sensor 12 outputs an output signal V2 expressed as V2=A2·sin(θ+90°)=A2·cos θ. Note that A1 and A2 each represent an amplitude.
If the amplitudes A1 and A2 are both regarded as being equal to A, or the output signals V1 and V2 are normalized such that the amplitudes A1 and A2 are equal to a predetermined value A, the output signal V1 is expressed as V1=A·sin θ, and the output signal V2 is expressed as V2=A·cos θ. Further, if A is 1(A=1), the output signal V1 is expressed as V1=sin θ, and the output signal V2 is expressed as V2=cos θ. Thus, for facilitating understanding on the following descriptions, the output signal V1 of the magnetic sensor 11 will be expressed as V1=sin θ, and the output signal V2 of the magnetic sensor 12 will be expressed as V2=sin(θ+90°)·cos θ. FIG. 5 illustrates how the output signals V1 and V2 of the respective magnetic sensors 11 and 12 change as the rotation angle θ of the rotor 1 changes.
The rotation angle θ of the rotor 1 may be determined according to, for example, Expression 1 shown below.
                                                        θ              =                            ⁢                                                tan                                      -                    1                                                  ⁡                                  (                                                            sin                      ⁢                                                                                          ⁢                      θ                                                              cos                      ⁢                                                                                          ⁢                      θ                                                        )                                                                                                        =                            ⁢                                                tan                                      -                    1                                                  ⁡                                  (                                                            sin                      ⁢                                                                                          ⁢                      θ                                                              sin                      ⁡                                              (                                                  θ                          +                                                      90                            ⁢                            °                                                                          )                                                                              )                                                                                                        =                            ⁢                                                tan                                      -                    1                                                  ⁡                                  (                                                            V                      ⁢                                                                                          ⁢                      1                                                              V                      ⁢                                                                                          ⁢                      2                                                        )                                                                                        Expression        ⁢                                  ⁢        1            
However, with the conventional rotation angle detection devices, such as the one described above, the rotation angle is not accurately calculated if one of the magnetic sensors comes to output abnormal output signals.